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WASHINGTON — With lawmakers from both parties clamoring for some kind of larger U.S. role in Syria’s civil war, President Barack Obama sought yesterday to slow a rush to judgment that regime forces have loosed chemical weapons on civilians, cautioning that “confirmation and strong evidence” of “this potential use” still are needed.

“We have to act prudently,” Obama said.

His guarded statements appeared to reflect what one U.S. intelligence official said were intelligence assessments of “low or moderate confidence” that regime forces had used the lethal nerve agent sarin on a small scale.

In U.S. intelligence analytical parlance, “moderate confidence” generally means that information lacks sufficient corroboration, while “low confidence” usually means that it’s too fragmented, it isn’t authenticated, and there are major concerns about the credibility of the sources.

Obama’s restraint also stemmed from lingering popular anger over his predecessor’s use of exaggerated intelligence to fan support for the 2003 U.S.-led invasion of Iraq, and his own reluctance to become embroiled in another war as he pulls U.S. combat forces out of Afghanistan.

CIA and Defense Intelligence Agency assessments agreed that there was insufficient evidence from tissue and soil samples to conclude concretely that forces loyal to Syrian President Bashar Assad had launched sarin against civilians, said someone who’s familiar with the issue.

The U.S. intelligence findings, made public on Thursday in letters sent to top lawmakers by the White House, follow more-definitively worded charges by Britain, France and Israel that Syrian forces have used chemical weapons. The White House letters stressed that harder evidence is required and said there were concerns about the “chain of custody” through which U.S. analysts obtained the “physiological samples” on which the assessments were based.

Despite the heavy caveats, the letters ignited calls by lawmakers of both parties for the president to make good on a pledge to intervene in Syria’s 2-year-old civil war if Assad crossed a “red line” by using chemical weapons. Most declined to outline specific steps that should be taken by Obama, who has limited his response to providing radios, night-vision goggles and other nonlethal equipment to moderate rebel forces, humanitarian aid to civilians, and organizational help to a coalition of moderate opposition leaders.

Speaking before talks yesterday with King Abdullah of Jordan, whose country is struggling to cope with more than 500,000 Syrian refugees, the president said his position hadn’t shifted.